Mercedes motorsport strategy director James Vowles has conceded the F1 team is unsure of what caused the pit stop delay for Valtteri Bottas at the Mexico City Grand Prix, with a gunman sustaining slight burns in the process.
Bottas was delayed by around 12 seconds during his second stop at the Autodromo Hermanos Rodriguez due to an issue with the front-left wheel, although his race had already been adversely influenced by a first-corner collision with Daniel Ricciardo.
It was the second case of a wheel causing a significant delay this season when being undone as the Finn's Monaco Grand Prix ended when the front-right was machined on.
Asked what had gone wrong at the stop, Vowles replied: “It's a good question and even as I talk to you now, we don’t have all of the answers.
"What I can tell you is that the wheel looked as though it was stuck on the axle.
"When the car came in the gunman did everything correctly and undid the nuts from the axle, as did all corners.
"But when they tried to remove the wheel, the wheel wouldn’t come off the axle anymore.
"They put the gun back on and gunned the nut on and back off again. It's a standard procedure to try and overcome some of the problems and even when they did that the wheel still wouldn’t come loose.
"The gunman then put his hand into the wheel and effectively brute-forced it loose off the axle and burned himself slightly in doing. It was a brave action by him.
"That was obviously costly in time but more importantly we need to understand why that wheel wasn’t able to come loose initially.
"There are some theories, potentially we had a little bit of debris and build up inside what we call the cake tin, the carbon area that sits inside the wheel.
"But as we talk to you now, we don’t understand that and there is an investigation over the next few days.”
Why Bottas was unable to pass Ricciardo
Bottas spent much of his race behind Ricciardo's McLaren before eventually bailing into the pits on two further occasions to take the fastest lap away from Red Bull's Max Verstappen.
On why the Finn was unable to overtake Ricciardo, Mercedes trackside engineering director Andrew Shovlin said: "When you are on the same aged tyres it is difficult.
"As we got closer he was sliding around. I think everyone was having to manage brake and power unit temperatures, sitting there lap after lap was a problem.
"Also, Ricciardo was able to manage his temperatures through the lap to get a good exit from the last corner and if you exit the last corner well, it is very difficult to launch an attack.
"These cars are quite tricky to follow when you are on a hot circuit, the overheating makes it awkward and whilst that was frustrating to endure with Valtteri, particularly when the car that put Valtteri in that position was the one that was then holding him up, we were perhaps grateful of that with Lewis because Sergio had a better car than Lewis but wasn't able to pass."